Document capture systems use a combination of a camera and a lighting element to acquire an image of a subject. Digital camera manufacturers use “zonal correction,” which divides an image into small zones and applies different corrections to each zone, to adjust the response of the camera based on the location within the image of each zone. This allows the camera to calibrate various non-uniformities captured by its sensor array such as gain, shading and lens falloff. However, illumination of the subject cannot be calibrated.
Specular reflections are mirror-like reflections of light from a document, which may cause non-uniformities in an image of a document. Camera-based document capture systems use multiple diffuse lamps to physically prevent specular reflections by minimizing glare on the subject. However, diffuse lamps are bulky, expensive and must be placed further away from the document stage to avoid specular reflections.